Embedded processors are widely used in a variety of applications and generally include processor circuitry that executes embedded code to perform desired functions. One type of embedded processor is a microcontroller which can be used, for example, to control the operation of a device, such as a motor. Another type of embedded processor is a digital signal processor (DSP) which can be used, for example, in a variety of communications products, such as cellular phones. The use of an embedded processor to perform desired functions generally requires the development and debug of the embedded code. In many applications, the embedded code includes foreground code for performing time critical tasks and background code for performing administrative or higher level tasks.
For certain applications, it can be particularly important to be able to debug the embedded code using real-time execution control. It can also be important to provide for real-time debug access to registers and memory in an embedded processor without using a debug monitor and without stopping the processor. Real-time execution control allows for the suspension of the embedded processor's execution of a given task while still allowing the processor to continue to service other, time critical tasks. Thus, real-time execution control allows the user of processor development and debug tools to interactively control the execution of embedded code within the system without necessarily interfering with the processor's ability to perform time critical tasks. For example, if embedded code is being debugged in a processor used to control a hard disk drive motor, the processor should not be allowed to stop controlling that motor. Otherwise, the motor may go out of control and destroy the hard disk drive. Thus, it is important to allow the processor to continue to execute the time critical task of controlling the motor while the embedded instruction code is being debugged.
One conventional execution control method is to stop all processor execution upon a break event and not allow for any interrupts to be processed until execution resumes. This approach is taken in stop mode emulation schemes used in some processors. However, this does not allow for controlling the processor's execution in a real-time, embedded system.
Another conventional method is to have a break event trigger a special interrupt which causes the processor to execute an interrupt service routine in which the processor waits for either a command to resume execution or for an enabled, time critical interrupt to occur. This type of interrupt service routine is often referred to as a "debug monitor." Thus, the debug monitor is implemented by code executed by the embedded processor after being placed in a debug state. This debug monitor approach provides a form of execution control and is used in some processors in addition to the use of stop mode emulation schemes.
In this debug monitor method, the special interrupt service routine typically communicates to a debug host through scanable registers. The debug host scans in commands and scans out results. When halted, the processor is actually inside the debug monitor, servicing time critical interrupt service routines while performing commands. Consequently, the debug monitor scheme suffers from problems in that it uses system resources such as program and data memory. In general, on chip memory is very expensive and possibly can be corrupted by the debug monitor. Further, performance overhead due to saving and restoring context is experienced as the debug monitor is entered and exited and time critical interrupts generally have to be blocked during this time period.